The presence of sulfur oxides and nitrogen oxides in emissions from combustion, i.e. fossile-fuel electric power plants, metallurgical plants and chemical plants, give rise to acid rain which tends to progressively increase the acidity in lakes, flowing surface water and ground water.
To an increasing extent, the acidification of the water damages the biosphere since in acidified waters, the greater part of the normally present water organisms can no longer survive.
Measurements have shown that in many waters, the sulphate ion concentration has already reached levels of 30-60 mg/l, i.e. concentrations well below the bicarbonate buffer level, which is effective to a pH as low as 5.5. The natural geological concentration of sulfate ion is about 5 mg per liter. In acidified water, the lower bicarbonate buffer pH threshold is readily passed by the high sulfate ion concentration which can give rise to a state, below a pH of about 5.0 to the aluminum buffer range, in which the water is in equilibrium with aluminum ion at aluminum concentrations which are toxic, i.e. of 0.1 mg of aluminum per liter or more. Such aluminum contents are known to be toxic to fish populations.
Recent studies have shown that not only lakes and streams, but even ground water is increasingly endangered by acidification.
The danger of acidification of waters is also recognized to extend many years beyond the original sulfur and nitrogen oxide emissions.
Accordingly, it has been proposed to neutralize excessively acidic portions of the hydrosphere with powdered lime.
A wide-area liming of surface waters, however, can give rise to a massive attack on sensitive components of the hydrosphere.
In order to effect a reasonable degree of neutralization of acid water, very large amounts of lime must be used since lime, because of its high density, rapidly falls to the bottom of the body of water being treated and is lost in a sediment formation or because it binds in a sludge sediment so that at least a portion of the lime is rendered ineffective for neutralization.
As a consequence, amounts up to 25 metric tons of lime have been required per hectare of the water surface. This is very expensive.
It has already been suggested, moreover, to spread lime on bodies of water from airplanes and helicopters. This, however, appears to have an adverse effect on the living populations of the waters treated. Apart frcm a significant liberation of carbon dioxide, utilizing these spreading techniques, there can be local increases of the pH value which can be detrimental for many water organisms. For example, there are sensitive breeds of fish which will be affected in this manner. As a matter of fact, there are few water-dwelling organisms which can tolerate a deviation from a pH range of 6 to 8.
Problems have been encountered in the use of metallurgical lime with its high calcium oxide content for the deacidification of waters. It appears that the calcium oxide reacts with water highly exothermally to liberate large amounts of heat which can also be detrimental.
It may be noted that it is not only the low pH which is detrimental to water-dwelling organisms, but that the increasing acidity is generally associated with increasing concentrations of nitrogen compounds in the water, especially from fertilizers,of cyanide compounds in the water, especially from metallurgical plants and electroplating plants, and increasing concentrations of chlorine and metal salts, from a variety of sources, especially iron, manganese, mercury and cadmium salts.